Acoustic resonators are used as oscillators in various electronic applications. An acoustic resonator can be characterized generally by a resonant frequency and acoustic coupling coefficient kt2. However, due to a variety of intrinsic and extrinsic influences, the resonant frequency is not stable.
One source of frequency drift in acoustic resonators is physical stress. Physical stress can be caused, for example, by forces transmitted to the acoustic resonator through adjacent components. As an example, an acoustic resonator can be formed on a substrate of a known material, for example silicon, and comprising components made from various materials. As the substrate is heated and/or cooled, the substrate may expand or contract unevenly because the various components have different temperature coefficients of expansion. This uneven expansion or contraction can cause the substrate to change shape in a “potato chip” fashion. As the substrate changes shape, the substrate can transfer forces to the acoustic resonator through various intervening components. As these forces are transferred to the acoustic resonator, they will change the resonant frequency of the acoustic resonator, and can deleteriously impact operation of an electronic device that includes the acoustic resonator.
What is needed, therefore, are techniques for reducing frequency drift due to physical stresses in acoustic resonator structures.